1. Technical Field
The present invention relates to a stabilized inorganic nanoparticle, a stabilized inorganic nanoparticle material, a method for producing a stabilized inorganic nanoparticle, and a method for using a stabilized inorganic nanoparticle.
The invention relates particularly to a stabilized inorganic nanoparticle obtained by bonding protective ligands to a surface of a nanoscale fine inorganic particle composed of a metal, a metal oxide, a semiconductor substance, etc. to stabilize the inorganic particle, which can be easily and rapidly functionalized by bonding functional ligands to the inorganic particle because the stabilized inorganic nanoparticle has a sufficient number of free binding sites on the surface and the protective ligands bonded is preferably high in substitution reactivity.
The invention further relates to a novel method for producing such a stabilized inorganic nanoparticle, and a method for using the stabilized inorganic nanoparticle for various purposes by bonding various functional ligand to characterize or functionalize the stabilized inorganic nanoparticle.
2. Background Art
[Metal Nanoparticle]
Inorganic nanoparticles, such as metal nanoparticles produced by forming metals such as gold into ultrafine particles, have unique chemical, electrical, and optical effects and catalytic activities, which cannot be found in bulk metals. Thus, researches have been made on the use of the inorganic nanoparticles in very many technological fields of photoelectrochemical devices, drug delivery systems, sensors, and the like and various developments on its application have been being contemplated.
However, the ultrafine metal nanoparticles are unstable without modifications, and are easily aggregated to form relatively large-diameter particles, which are not nanoparticles. This is an important disadvantage of the metal nanoparticles in practical use. Thus, the metal nanoparticle are, for example, poor in storage stability, and have to be used immediately after their preparation. Further, after the preparation of the metal nanoparticles, it is difficult or impossible to characterize them before using.
[Stabilization of Metal Nanoparticle]
In the ultrafine metal nanoparticles, a large number of metal atoms forming the particle are disposed on the particle surface, and can form bonds with various functional groups such as thiol, disulfide, phosphine, and amine groups. Thus, methods of producing a stabilized metal nanoparticle having a good stability (storage stability), which contains using the metal atom on the metal nanoparticle surface as a binding site, and bonding a protective ligand for stabilizing to the binding site, thereby modifying the metal nanoparticle, have been proposed.
[Reference 1] Mathias Brust, Merryl Walker, Donald Bethell, David J. Schiffrin, and Robin Whyman, “Synthesis of Thiol-derivatised Gold Nanoparticles in a Two-phase Liquid-Liquid System”, Journal of Chemical Society-Chemical Communications, 801-802 (1994)
[Reference 2] M. Brust, J. Fink, D. Bethell, D. J. Schiffrin, and C. Kiely, “Synthesis and Reactions of functionalized Gold Nanoparticles”, Journal of Chemical Society-Chemical Communications, 1655-1656 (1995)
For example, Brust et al. have proposed a method of preparing gold nanoparticles and stabilizing the gold nanoparticles by using a protective ligand (a thiol compound) in References 1 and 2. The essential point of the method is such that AuCl4− is reduced under presence of an aqueous NaBH4 solution in a toluene solution to generate gold nanoparticles, and the toluene solution contains protective ligands such as n-dodecanethiol for stabilizing the metal clusters and a phase transfer agent of tetraoctylammonium.
Further, it has been reported that the gold nanoparticles prepared by this method have a narrow particle diameter distribution range. It is known that the particle sizes of the metal nanoparticles greatly affect various properties thereof, and thus the narrow particle diameter distribution range is regarded as preferable.
Teranishi et al. have reported in the following References 3 and 4 that gold nanoparticles having a remarkably narrow particle diameter distribution, protected by thiol compounds, can be obtained by treating a solid sample prepared beforehand at a controlled temperature.
[Reference 3] T. Teranishi, S. Hasegawa, T. Shimizu, and M. Miyake, “Heat-Induced Size Evolution of Gold Nanoparticles in the Solid State”, Adv. Mater., 13, 1699-1701 (2001)
[Reference 4] T. Shimizu, T. Teranishi, S. Hasegawa, and M. Miyake, “Size Evolution of Alkanethiol-protected Gold Nanoparticles by Heat Treatment in the Solid State”, Journal of Physical Chemistry B, 107, 2719-2724 (2003)
Various functional groups can be bonded to the metal nanoparticle surfaces as described above, and the greatest benefit thereof is not that the protective ligands for stabilizing the nanoparticles can be bonded to the surfaces, but that the metal nanoparticles are functionalized, namely various molecules with various characteristics and functions (functional ligands) can be bonded to the surfaces to functionalize the metal nanoparticle. By the functionalization, the resultant metal nanoparticles can show the physicochemical properties of the functional ligands or additional properties, whereby it becomes possible to use the metal nanoparticles for further greater range of applications.
In the report by Brust, et al., in addition to the stabilization of the gold nanoparticles, functionalization thereof by replacing the protective ligand with a functional ligand is described. However, in the case of the stabilized gold nanoparticles according to the report by Brust, et al., it generally takes 2 days or more to sufficiently replace the protective ligands of dodecanethiol by the functional ligands, and the functionalization cannot be expected to be practically used due to the inefficiency.
It has been proposed that protective ligands poor in bonding strength, such as triphenylphosphine, amine, and tert-dodecanethiol, are used instead of dodecanethiol to accelerate the substitution with the functional ligands. However, as a result of experiments by the inventors, the substitution is not accelerated very much by using such protective ligands. Thus, it seems difficult to solve the problem by using such protective ligands instead.
[Reference 5] M. Montalti, L. Prodi, N. Zaccheroni, and G. Battistini, “Modulation of the Photophysical Properties of GoldNanoparticlesbyAccurate Control of the Surface Coverage”, Langmuir, 2004, 20, 7884-7886
In above Reference 5, a study on controlling coverage of gold nanoparticles with a fluorescent molecular is disclosed. However, the study is made in view of fluorescence switching, and the bonding of the fluorescent molecular to the gold nanoparticles is not for purpose of stabilizing and functionalizing the gold nanoparticles. Further, the density of the fluorescent molecules on the gold nanoparticle surfaces is controlled only by selecting the amount of the fluorescent molecular added to the reaction system, and as shown in FIG. 1 of Reference 5, a distinguishing relation is not observed between the surface density of the fluorescent molecular and stabilization/functionalization of the gold nanoparticles.
Though the above problems are described with respect to metal nanoparticles, inorganic nanoparticles composed of inorganic materials other than metals, such as metal oxides and semiconductor materials, have the same problems.